This invention relates to battery chargers, and more particularly to battery charging methods.
Battery manufacturers use the rise in temperature per unit of time when charging batteries. If one looks at the change in temperature across time, the graphical slope characteristic, known as dT/dt; may be used to determine the battery""s charge capacity. As the battery fills with energy, the rise in temperature per second increases. Thus, the dT/dt measurement can be used to cause the charger to switch from rapid charge to trickle charge once a threshold change in temperature slope is detected. This dT/dt termination technique is based on the concept that batteries stay at substantially the same temperature during the charge sequence, and once fully charged, become exothermicxe2x80x94that is, the extra current going into the battery is no longer accepted and turns into heat. Thus, the rise in battery temperature determines when the battery is fully charged.
Two inherent problems exist with this approach. First, batteries that have separate battery contacts for the radio (discharge) and charger (charge) paths, typically contain a reverse discharge diode (in the charge path). This diode is to ensure that an external short of the external contacts won""t spark or accidentally drain the battery. The problem with this diode is that it causes temperature problems in the battery. When the charge sequence starts, the rise in temperature created by the diode gets coupled to the cell temperature and can cause the charger to falsely dT/dt. Accordingly, the battery flex circuit must be designed to keep the diode far enough away from the thermistor (which is used to read the battery temperature) to avoid this problem. This typically requires a more design intensive and expensive flex design.
The second problem is that NiMH (nickel-metal hydride) batteries have exothermic characteristics and have a steep temperature slope at the start of charge and at the end of charge. This initial steep temperature slope at the start of charge can cause the charger to falsely terminate rapid charge by reading a misleading dT/dt trip point.
Accordingly, there is a need for an improved battery charging method which properly terminates rapid charge, especially for batteries that contain an additional reverse discharge protection diode, and for NiMH batteries that act exothermic especially at the beginning of the charge sequence.